How to pattern match Ecto query error

Question

Like other functions in elixir (as well as Ecto's own transactions), I want to pattern match to handle potential errors with Ecto queries. Like this:

case Repo.get!(User, id) do
  {:ok, user} ->
    #do something
  {:error, message} ->
    #pass the error
end

Obviously this does not work, but how can I pattern match Ecto errors like Ecto.NotSingleResult and other potential query problems like preload errors?

Answer 1

Use Repo.get which will return a value or nil. You can then pattern match on the expected struct or use if-clauses. Repo.get! raises on purpose (for the cases you expect a struct to be there and not being there is an error).

Answer 2

This might do the trick

case Repo.get(User, id) do
  user when is_map(user) -> {:ok, user}
  nil -> {:error, "not found"}
end

Reffering to this Elixir, Ecto pattern matching conditional with db query not behaving as expected